Method for Cooling in Distillation and Polymerisation Process by Absorption Refrigeration

ABSTRACT

Processes for the separation of a hydrocarbon-containing feed streams are described herein. The processes generally include cooling the hydrocarbon-containing feed stream using a first absorption refrigeration cycle to form a cooled feed stream, introducing the cooled feed stream into a first distillation zone for subjecting the cooled feed stream to distillation conditions adapted to remove a first bottom stream including co-monomer and a first overhead stream including hydrocarbon diluents, olefin monomer and further components selected from H 2 , N 2 , O 2 , CO, CO 2 , formaldehyde and combinations thereof, passing the overhead stream from the first distillation zone to a second distillation zone for subjecting the overhead stream to distillation conditions adapted to remove a second bottom stream including substantially olefin-free hydrocarbon diluents, a side stream including hydrocarbon diluent and a second overhead vapor stream including olefin monomer, diluents and further components selected from H 2 , N 2 , O 2 , CO, CO 2 , formaldehyde and combinations thereof, cooling the second overhead vapor stream using a second absorption refrigeration cycle to form a cooled overhead vapor stream and separating olefin monomer from diluents in the cooled overhead vapor stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT/EP2008/059386, filed Jul. 17,2008, which claims priority from EP 07112962.1, filed Jul. 23, 2007.

FIELD OF THE INVENTION

The present invention relates generally to olefin polymerization. Inparticular, the present relates to a process for the separation of ahydrocarbon-containing feed stream comprising olefin monomer, co-monomerand hydrocarbon diluent. The present invention also relates to a processfor optimizing the separation of an overhead vapor stream comprisingolefin monomer and diluent.

BACKGROUND OF THE INVENTION

Polyolefins such as polyethylene and polypropylene may be prepared byparticle form polymerization, also referred to as slurry polymerization.

Olefin polymerizations are frequently carried out using monomer, diluentand catalyst and optionally co-monomers and hydrogen in a reactor. Thepolymerization is usually performed under slurry conditions, wherein theproduct consists usually of solid particles and is in suspension in adiluent. The slurry contents of the reactor are circulated continuouslywith a pump to maintain efficient suspension of the polymer solidparticles in the liquid diluent. The diluent does not react but istypically utilized to control solids concentration and also to provide aconvenient mechanism for introducing the catalyst into the reactor.Following such polymerization process a polymerization effluent isproduced comprising slurry of polymer solids in a liquid that containsdiluent, dissolved unreacted monomer, and dissolved unreactedco-monomer. Typically this liquid also includes traces of heavierelements, e.g. oligomers, and lighter components including H₂, N₂, O₂,CO and/or CO₂. Catalyst will generally be contained in the polymer.

The product is further discharged to a flash tank, through flash lines,where most of the diluent and unreacted monomers are flashed off.Afterwards, it is highly desirable to further treat the vapors in orderto recover the unreacted monomer, unreacted co-monomer and the diluent,since there is an economic interest in re-using these separatedcomponents including the monomer, co-monomer, and the diluent, in apolymerization process. Alternatively, the product slurry may be fed toa second loop reactor serially connected to the first loop reactor wherea second polymer fraction may be produced. Typically, when two reactorsin series are employed in this manner, the resultant polymer product,which comprises a first polymer fraction produced in the first reactorand a second polymer fraction produced in the second reactor, has abimodal molecular weight distribution.

Slurry polymerization in a loop reaction zone has proven commerciallysuccessful. The slurry polymerization technique has enjoyedinternational success with billions of pounds of polyolefins being soproduced annually.

A variety of equipment and operations within a polyolefin manufacturingprocess may consume energy. Noteworthy consumers of electricity within apolyolefin plant, for example, may include the pumps that circulate theliquid reaction mixture in the polymerization reactors (e.g., loopslurry reactors), the pumps that circulate the cooling medium (e.g.,treated water) through the polymerization reactor jackets, thecompressors that pressurize and return recycled diluent (and/or monomer)to the polymerization reactor, the blowers used to convey fluff andpellets, and the extruders that convert the polyolefin fluff topolyolefin pellets. Significant users of steam in a typical polyolefinplant may include heaters that flash liquid in the effluent of thepolymerization reactor, and fractionation columns that process recovereddiluent and/or monomer. Relatively large consumers of fuel gas mayinclude activation processes (which may utilize high heat) of thepolymerization catalyst, and operations that maintain adequatecombustible content in the plant flare header (in the feed to theflare). In general, extensive energy is required to polymerize themonomer and comonomer to polyolefin fluff, to process recycled effluentfrom the reactor, and to convert the polyolefin fluff to pellets.

Therefore, the production of polyolefin is an energy-intensive process,consuming electricity, steam, fuel gas, and so on. Such energyconsumption generally contributes significant cost to the production ofpolyolefins, as well as to the downstream products constructed ofpolyolefins distributed to the customer.

SUMMARY OF THE INVENTION

The present invention is primarily directed to a process for improvingthe separation of a vaporous hydrocarbon containing stream separatedfrom the effluent from a homo-polymerization and/or co-polymerizationprocess.

In particular, the present invention concerns a process for theseparation of a hydrocarbon-containing feed stream (20) comprising thesteps of:

a) optionally cooling said hydrocarbon-containing feed stream (20) usingan absorption refrigeration cycle (37),b) introducing said feed stream (20) into a first distillation zone (22)for subjecting said feed (20) to distillation conditions adapted toremove b1) a bottom stream (25) comprising co-monomer, and b2) anoverhead stream (29) comprising hydrocarbon diluent, olefin monomer andfurther components such as H₂, N₂, O₂, CO, CO₂, and formaldehyde, andc) introducing the overhead stream (29) of step b) in a seconddistillation zone (23) for subjecting said stream (29) to distillationconditions adapted to remove c1) a bottom stream (30) comprisingsubstantially olefin-free hydrocarbon diluent, c2) a side stream (31)comprising hydrocarbon diluent, and c3) an overhead vapor stream (32)comprising olefin monomer, diluent and further (lighter) components suchas formaldehyde, H₂, N₂, O₂, CO and CO₂, andd) cooling the temperature of said removed overhead vapor stream (32) ofstep c3) using an absorption refrigeration cycle (34) prior toseparating (35) said olefin monomer from said diluent in said overheadvapor stream (32).

The inventors have surprisingly found that energy efficiency in theproduction of polyolefin could be increased.

The present invention allows the use of thermal energy produced on anolefin polymerization site, wherein said energy is recycled back to saidpolymerization site. This invention relates to the efficient energyrecovery of the heat produced by reboiler of distillation unit fordriving absorption refrigeration cycle used for refrigerating streamremoved from said distillation unit in a polyolefin production site.

In an embodiment of said process, the temperature of said overhead vaporstream (32) is cooled to a temperature between 0° C. and −40° C.,preferably between −10° C. and −30° C., preferably between −10° C. and−20° C. Preferably, water and ammonia are used as working fluids in saidabsorption refrigeration cycle (34) of step d).

In another embodiment, said absorption refrigeration cycle (34) of stepd) is driven using heat recycled from a reboiler of the firstdistillation zone (22).

In some embodiments of the foregoing process, the temperature of theside stream (31) of step c2) is also cooled using an absorptionrefrigeration cycle. Preferably, water and lithium bromide are used asworking fluids in the absorption refrigeration cycle of step c2).Alternatively, water and ammonia can be used as working fluids in theabsorption refrigeration cycle of step c2).

In some embodiments of the foregoing process, water and lithium bromideare also used as working fluids in the absorption refrigeration cycle ofstep a).

Some embodiments of the foregoing process, comprise introducing thebottom stream (25) of step b) in a third distillation zone forsubjecting said bottom stream (25) to distillation conditions adapted toremove 1) a bottom stream comprising co-monomer and 2) an overheadstream comprising hydrocarbon diluent.

The present invention also concerns a process for the production ofpolyolefins comprising the steps of

-   -   introducing into a reactor one or more olefin reactants,        polymerization catalysts and diluents, and while circulating        said reactants, catalysts and diluents,    -   polymerizing one or more olefin reactants to produce a polymer        slurry comprising essentially liquid diluent and solid olefin        polymer particles,    -   recovering olefin polymer particles from the slurry by        separating at least a majority of the diluent from the slurry in        a hydrocarbon-containing feed stream,    -   distilling and separating said hydrocarbon-containing feed        stream according to a process of separation as presently        claimed.

In an embodiment said distillation is optimized using a process asdescribed herein.

The present invention also concerns the use of a separation processaccording to the present invention in a polyolefin production process,comprising the steps of

-   -   introducing into a reactor one or more olefin reactants,        polymerization catalysts and diluents, and while circulating        said reactants, catalysts and diluents,    -   polymerizing one or more olefin reactants to produce a polymer        slurry comprising essentially liquid diluent and solid olefin        polymer particles, and    -   recovering olefin polymer particles from the slurry by        separating at least a majority of the diluent from the slurry in        a hydrocarbon-containing feed stream (20), and        separating said hydrocarbon-containing feed stream (20) using a        separation process as described herein.

In another aspect, the present invention also concerns a polyolefinproducing unit, comprising

-   -   means for feeding monomer, a co-monomer, diluent, a        polymerization catalyst and optionally hydrogen to at least one        polymerization reactor;    -   a reactor system comprising at least one polymerization reactor        defining a flow path for a polymer slurry,    -   one or more diluent/monomer recovery system configured to        separate a majority of the diluent from the slurry discharged        from the polymerization reactor;    -   one or more fractionation system configured to process a portion        of the diluent discharged from the diluent/monomer recovery        system and to provide recovered diluent substantially free of        olefin monomer; and    -   a polyolefin processing system configured to process polyolefin        particles recovered from the slurry in the diluent/monomer        recovery system,        wherein said one or more fractionation system is coupled with        one or more absorption refrigeration unit.

The present invention provides a process that allows the optimizing adistillation process in a polyolefin production plant, comprising thesteps of recovering an overhead vapor stream from a distillation unitand cooling the temperature of said overhead vapor stream using anabsorption refrigeration cycle thereby optimizing the distillationprocess.

The present invention allows reducing energy consumption in a polyolefinproduction process and an increase of the hydrocarbon recovery. Thepresent invention improves plant efficiency and permits to recoverwasted energy resources.

The present invention will be further disclosed in detail hereunder. Thedescription is only given by way of example and does not limit theinvention. The reference numbers relate to the hereto-annexed figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically represents a separation unit comprising aseparation column coupled with an absorption refrigeration unitaccording to an embodiment of the present invention.

FIG. 1B schematically represents a separation unit comprising aseparation column coupled with an absorption refrigeration unitaccording to another embodiment of the present invention.

FIG. 2A schematically represents a separation unit comprising twodistillation columns, one of which is coupled to an absorptionrefrigeration unit according to an embodiment of the present invention.

FIG. 2B schematically represents a separation unit comprising twodistillation columns, one of which is coupled to an absorptionrefrigeration unit according to another embodiment of the presentinvention.

FIG. 3 schematically represents a separation unit comprising threedistillation columns, one of which is coupled to an absorptionrefrigeration unit according to an embodiment of the present invention.

DETAILED DESCRIPTION

The invention provides a system for energy valuation in a separationprocess of a vaporous hydrocarbon stream, wherein at least oneabsorption refrigeration cycle is thermally coupled to the separationprocess of said vaporous hydrocarbon stream. In a preferred example,such vaporous hydrocarbon stream may be issued from the effluent of apolymerization process, in particular for the polymerization ofethylene. Advantageously, application of the present separation processincludes the benefits of less energy usage.

The present invention concerns a process for the separation of ahydrocarbon-containing feed stream comprising olefin monomer, co-monomerand hydrocarbon diluent, comprising the steps of: distilling saidhydrocarbon-containing feed stream, removing a bottom stream comprisingsubstantially olefin-free hydrocarbon diluent, optionally removing aside stream comprising hydrocarbon diluent, removing an overhead vaporstream comprising olefin monomer and diluent, and cooling thetemperature of said removed overhead vapor stream using an absorptionrefrigeration cycle prior to separating said olefin monomer and diluentin said vapor stream.

Cooling this stream minimizes the diluent in this stream, and maximizesdiluent recovery.

In another aspect, the present invention concerns a process foroptimizing the separation of a hydrocarbon-containing feed streamcomprising olefin monomer, co-monomer and hydrocarbon diluent,comprising: distilling said hydrocarbon-containing feed stream, removinga bottom stream comprising substantially olefin-free hydrocarbondiluent, optionally removing a side stream comprising hydrocarbondiluent, and removing an overhead vapor stream comprising olefin monomerand diluent from said distillation step, wherein the optimizationcomprises cooling the temperature of said hydrocarbon-containing feedstream, using an absorption refrigeration cycle prior to saiddistillation.

In this embodiment, the hydrocarbon-containing feed stream is cooledbefore the compressor, upstream of the distillation section. Coolingbefore the compressor allows a downsizing of the compressor and reducesthe electrical consumption.

The present processes allow the optimization of the separation of anoverhead vapor stream comprising olefin monomer and diluent, wherein theprocess comprises the steps of distilling a hydrocarbon-containing feedstream and removing the overhead vapor stream comprising olefin monomerand diluent from said distillation, wherein the optimization comprisescooling the temperature of said removed overhead vapor stream using anabsorption refrigeration cycle thereby optimizing the separation of saidmonomer from said diluent.

In another aspect, the present invention concerns a polyolefinproduction process, comprising the steps of

-   -   introducing into a reactor one or more olefin reactants,        polymerization catalysts and diluents, and while circulating        said reactants, catalysts and diluents,    -   polymerizing one or more olefin reactants to produce a polymer        slurry comprising essentially liquid diluent and solid olefin        polymer particles,    -   recovering olefin polymer particles from the slurry by        separating at least a majority of the diluent from the slurry in        a hydrocarbon-containing feed stream,    -   distilling said hydrocarbon-containing feed stream,    -   removing an overhead vapor stream comprising olefin monomer and        diluent from said distillation step, and    -   cooling the temperature of said removed overhead vapor stream        using an absorption refrigeration cycle prior to separating said        olefin monomer and diluent in said removed vapor stream.

Preferably, said process comprises the steps of cooling the temperatureof said hydrocarbon-containing feed stream, using an absorptionrefrigeration cycle prior to said distillation. In an embodiment, saidremoved overhead vapor stream is further distilled, and a secondoverhead vapor is removed from said second distillation prior to coolingsaid second overhead vapor stream.

The present invention therefore provides a process for optimizing theseparation of an overhead vapor stream comprising olefin monomer anddiluent, wherein an hydrocarbon-containing feed stream is distilled andsaid overhead vapor stream comprising olefin monomer and diluent isremoved from said distillation step, wherein the optimization consistsin cooling the temperature of said removed overhead vapor stream usingan absorption refrigeration cycle thereby optimizing the separation ofsaid monomer from said diluent. In an embodiment, said removed overheadvapor stream is further distilled, and a second overhead vapor isremoved from said second distillation prior to cooling said secondoverhead vapor stream using said absorption refrigeration unit.

In an embodiment, said process comprises cooling the temperature of saidoverhead vapor stream between 0° C. and −40° C., preferably between −10°C. and −30° C., more preferably between −10° C. and −25° C., yet morepreferably between −10° C. and −20° C., more preferably at about −20° C.

In an embodiment, said absorption refrigeration cycle is driven usingheat recycled from a reboiler of a distillation column. In anembodiment, said boiler is the boiler of a first distillation column.

According to a particular embodiment, at least one absorptionrefrigeration cycle is in thermal contact with the overhead vapor streamremoved from a separation column. The absorption refrigeration unittypically comprises one or more cooling circuits, each comprising agenerator, a condenser and an evaporator, in association with anexpansion valve.

The hydrocarbon-containing feed stream that is to be separated accordingto the present invention in such distillation system will generally bean overhead stream coming from a flash tank and purge columns of apolymerization reactor, wherein a stream containing solvent, polymer andunreacted monomers is flashed or otherwise treated to remove solvent ordiluent and monomers there from. In an embodiment, said flashed ortreated feed stream is cooled using at least one absorptionrefrigeration unit prior to said distillation. In an embodiment, saidhydrocarbon-containing feed stream is cooled using two absorptionrefrigeration units.

A presently preferred component stream separated according to theinvention comprises monomer, such as ethylene, co-monomer, such as1-hexene, and diluent, such as isobutane. It should be recognizedhowever, that the distillation system of the invention is equallyapplicable to other monomer, co-monomer and diluent systems so long asfeed vapors comprise hydrocarbons, which permit separation bydistillation. Traces of both heavier, e.g. oligomers, and lightercomponents such as formaldehyde, N₂, H₂, and components such as O₂, COand CO₂ are generally also present in such effluent streams.

In another embodiment said hydrocarbon-containing feed stream can be anoverhead stream coming from a first distillation column.

A typical absorption refrigeration cycle uses at least two substances:ammonia and water is a typical pair of working fluids, but other pairsof working fluids can be used. As used herein, the term “working fluid”refers to the medium evolving within a thermodynamic cycle.

In an embodiment, the absorption refrigeration unit cools the overheadvapor stream removed from a distillation column by evaporating liquidammonia in an optional hydrogen environment. The now-gaseous ammonia isthen absorbed (dissolved) into water, and then later separated (boiledoff from the water) by a small source of heat (in the form of steam orhot water from the distillation column reboiler). This drives off thedissolved ammonia gas which is then condensed into a liquid. The liquidammonia then enters the evaporator to repeat the cycle.

Suitable working fluids for said absorption refrigeration cycle besidesthe preferred water and ammonia mixture include a circulating materialthat undergoes a phase change to promote the absorption and release ofheat energy such as for example a solution of lithium bromide salt andwater. Water is evaporated under low pressure from the coils that arebeing chilled. The water is absorbed by a lithium bromide/watersolution. The water is driven off the lithium bromide solution usingheat.

In a preferred embodiment, the absorption refrigeration unit for use inthe invention uses substances: ammonia, and water and optionallyhydrogen gas. The cooling cycle starts in an evaporator, where liquefiedanhydrous ammonia enters. The evaporator may contain another gas(preferably hydrogen), which presence may help lowering the partialpressure of the ammonia in that part of the system. The lowered partialpressure of ammonia changes the ammonia's boiling point, bringing it lowenough that it can boil below room temperature. The overhead vaporstream in thermal contact with said ammonia is cooled in saidevaporation upon boiling of said ammonia.

Said ammonia is then sent to an absorber comprising a downhill flow oftubes in which the mixture of gases flows in contact with water beingdripped from above. Once the water reaches the bottom, it is thoroughlymixed with the ammonia, and the hydrogen can flow freely back to theevaporator. Hot water or steam from the distillation column reboiler isused for separating ammonia from water in the generator. Ammonia gas isthen conveyed in a separator where it is dried and the water is recycledback through the previous absorption step. The next step is thecondenser where the hot ammonia gas is cooled back down to roomtemperature. Because of the pressure, the ammonia condenses back into aliquid and the cycle starts over again.

One of the advantages of using an absorption refrigeration unit at thisstage of the feed recycling process is that said absorptionrefrigeration unit utilizes a heat source available in the vicinity ofthe unit (a reboiler for distillation column) to provide the energyneeded to drive the cooling system rather than being dependent onelectricity to run a compressor.

In another aspect, the present invention provides a process for theseparation of a hydrocarbon-containing feed stream comprising olefinmonomer, co-monomer and hydrocarbon diluent, comprising the steps of:distilling said hydrocarbon-containing feed stream, removing a bottomstream comprising substantially olefin-free hydrocarbon diluent,optionally removing a side stream comprising hydrocarbon diluent,removing an overhead vapor stream comprising olefin monomer and diluent,and cooling the temperature of said removed overhead vapor stream usingan absorption refrigeration cycle prior to separating said olefinmonomer and diluent in overhead said vapor stream.

In an embodiment, the temperature of said removed overhead vapor streamis cooled between 0° C. and −40° C., preferably between −10° C. and −30°C., more preferably between −10° C. and −25° C., yet more preferablybetween −10° C. and −20° C., more preferably at about −20° C.

The advantage of the whole process is that said absorption refrigerationcycle is driven using heat recycled from a reboiler of a distillationcolumn.

In an embodiment, said hydrocarbon-containing feed stream comprisingolefin monomer, co-monomer and hydrocarbon diluent, is an overhead vaporstream removed from a first distillation column.

In an embodiment, the temperature of said hydrocarbon-containing feedstream comprising olefin monomer, co-monomer and hydrocarbon diluent, iscooled using an absorption refrigeration cycle. In particular, saidcooling is performed prior to compressing said hydrocarbon-containingfeed stream. In a preferred embodiment said hydrocarbon-containing feedstream is cooled using a first refrigeration cycle, said cooledhydrocarbon-containing feed stream is then compressed and further cooledusing a second refrigeration cycle before being further compressed. Inan embodiment, water and lithium bromide are used as working fluids insaid absorption refrigeration cycle.

In an embodiment, the temperature of said removed side stream is cooledusing an absorption refrigeration cycle. In particular, said cooling isperformed prior to compressing said removed side stream. In anembodiment, water and lithium bromide are used as working fluids in saidabsorption refrigeration cycle.

In a further aspect, the present invention provides a process foroptimizing the separation of a hydrocarbon-containing feed streamcomprising olefin monomer, co-monomer and hydrocarbon diluent,comprising: distilling said hydrocarbon-containing feed stream, removinga bottom stream comprising substantially olefin-free hydrocarbondiluent, optionally removing a side stream comprising hydrocarbondiluent, and removing an overhead vapor stream comprising olefin monomerand diluent, wherein the optimization consists in cooling thetemperature of said hydrocarbon-containing feed stream using anabsorption refrigeration cycle prior to said distillation.

In an embodiment, water and lithium bromide are used as working fluidsin said absorption refrigeration cycle. In particular, said cooling isperformed prior to compressing said hydrocarbon-containing feed stream.In a preferred embodiment said hydrocarbon-containing feed stream iscooled using a first refrigeration cycle, said cooledhydrocarbon-containing feed stream is then compressed and further cooledusing a second refrigeration cycle before being further compressed andbefore being distilled.

In an embodiment, the removed overhead vapor stream is cooled using anabsorption refrigeration cycle.

In another embodiment, the removed overhead vapor stream is furtherdistilled in a second distillation unit and a second overhead vaporstream is further removed from said second distillation step. In anembodiment, the second removed overhead vapor stream is cooled using anabsorption refrigeration cycle. Preferably, water and ammonia are usedas working fluid for the cooling of said overhead vapor stream.

The advantage of the process is that said absorption refrigeration cycleis driven using heat recycled from a reboiler of a distillation column,for example of said first distillation column.

The present invention is applicable to any process producing an effluentcomprising a slurry of particulate polymer solids suspended in a liquidmedium comprising a diluent and unreacted monomer. Such reactionprocesses include those which have come to be known in the art asparticle form polymerizations.

More in particular, the present invention relates to a separationprocess of a hydrocarbon-containing feed, wherein saidhydrocarbon-containing feed stream comprising olefin monomer, co-monomerand hydrocarbon diluent is an effluent stream obtained from apolymerization process for preparing polyethylene, and preferably forpreparing monomodal or bimodal polyethylene. Ethylene polymerizes in aliquid diluent in the presence of a catalyst, optionally a co-catalyst,optionally a co-monomer, optionally hydrogen and optionally otheradditives, thereby producing polymerization slurry. In a preferredembodiment, present invention is particularly suitable for thepolymerization of ethylene in isobutane diluent.

Suitable ethylene polymerization includes but is not limited tohomopolymerization of ethylene, copolymerization of ethylene and ahigher 1-olefin co-monomer such as 1-butene, 1-pentene, 1-hexene,1-octene or 1-decene. In an embodiment of the present invention, saidco-monomer is 1-hexene. In a preferred embodiment, the present inventionis directed to the separation process of a vaporous stream, which isissued from the effluent of an ethylene polymerization reaction whereinreactants including the monomer ethylene, isobutane as hydrocarbondiluent, a catalyst, the co-monomer 1-hexene and hydrogen are used.However, it will be appreciated that the present processes areapplicable to separate a vaporous stream, which is issued from theeffluent of any other polymerization reaction involving other monomer,co-monomer and diluent systems as long as the feed vapors comprisehydrocarbons which permit separation by distillation.

More in particular, the present invention relates to a separationprocess of a hydrocarbon-containing feed, wherein saidhydrocarbon-containing feed stream comprising olefin monomer, co-monomerand hydrocarbon diluent is an effluent stream obtained from apolymerization process for preparing polyethylene, and preferably forpreparing monomodal or bimodal polyethylene. Preferably, separatedmonomer, hydrocarbon diluent and co-monomer are re-used in saidpolymerization process. “Bimodal PE” refers to PE that is manufacturedusing two reactors, which are connected to each other in series, theoperating conditions being different in the two reactors. “Monomodal PE”is produced in a single reactor or using two reactors in series, withidentical operating conditions.

As used herein, the term “polymerization slurry” or “polymer slurry” or“slurry” means substantially a multi-phase composition including atleast polymer solids and a liquid phase and allows for a third phase(gas) to be at least locally present in the process, the liquid phasebeing the continuous phase. The solids include catalyst and apolymerized olefin, such as polyethylene. The liquids include an inertdiluent, such as isobutane, dissolved monomer such as ethylene,co-monomer, molecular weight control agents, such as hydrogen,antistatic agents, antifouling agents, scavengers, and other processadditives.

Preferably, separated monomer, hydrocarbon diluent and co-monomer arere-used in said polymerization process.

The present invention is particularly suitable for polymerizationprocess for the manufacture of particulate olefin polymers consisting ofthe catalytic polymerization of olefins such as C₂ to C₈ olefins in adiluent containing the monomer to be polymerized, the polymerizationslurry being circulated in a loop reactor to which the starting materialis fed and from which the polymer formed is removed. Examples ofsuitable monomers include but are not limited to those having 2 to 8carbon atoms per molecule, such as ethylene, propylene, butylene,pentene, butadiene, isoprene, 1-hexene and the like.

The polymerization reaction can be carried out at a temperature of from50 to 120° C., preferably at temperature of from 70 to 115° C., morepreferably at temperature of from 80 to 110° C., and at a pressure offrom 20 to 100 bar, preferably at pressure of from 30 to 50 bar, morepreferably at pressure of 37 to 45 bar.

Suitable diluents are well known in the art and include but are notlimited to hydrocarbon diluents such as aliphatic, cycloaliphatic andaromatic hydrocarbon solvents. The preferred solvents are C₁₂ or lower,straight chain or branched chain, saturated hydrocarbons, C₅ to C₉saturated alicyclic or aromatic hydrocarbons. No limiting illustrativeexamples of solvents are butane, isobutane, pentane, hexane, heptane,cyclopentane, cyclohexane, cycloheptane, methyl cyclopentane, methylcyclohexane, isooctane, benzene, toluene, and xylene. In a preferredembodiment of the present invention, said diluent is isobutane. However,it should be clear from the present invention that other diluents may aswell be applied according to the present invention.

Suitable catalysts are well known in the art. According to the presentinvention, the term “catalyst” is defined herein as a substance thatcauses a change in the rate of a co-polymerization reaction withoutitself being consumed in the reaction. Examples of suitable catalystsinclude but are not limited to chromium oxide such as those supported onsilica or aluminum, organometal catalysts including those known in theart as “Ziegler” or “Ziegler-Natta” catalysts, metallocene catalysts andthe like. The term “co-catalyst” as used herein refers to materials thatcan be used in conjunction with a catalyst in order to improve theactivity of the catalyst during the polymerization reaction.

The terms “distillation system” or “separation system”, “recoverysystem”, are used in some embodiment of the present invention assynonyms and refer to systems comprising all necessary equipment adaptedto separate and recover unreacted reactants form the effluent stream ofa polymerization reaction. Such recovery systems generally include oneor more distillation columns. The term “distillation zone”, “separationcolumn” and “distillation column” may be used herein as synonyms. In apreferred embodiment, the present distillation process is carried out ina distillation system, which comprises one or more distillation zones orcolumns.

In a preferred embodiment, one or more of said distillation columns aretray columns. Such tray columns comprise a number of trays of variousdesigns to hold up the liquid in order to provide better contact betweenvapor and liquid. Trays essentially act as a unit operation, eachaccomplishing a fraction of the separation between liquid and gas. It isclear that the more trays there are, the better the degree ofseparation, and thus the better column performance will be. However,using a large number of trays in distillation columns has importantdisadvantages, especially with regard to construction. Suitabledistillation systems comprise distillation system having column(s) witha low number of trays, preferably lower than 25, even more preferredlower than 20. Nevertheless, although distillation columns with a lownumber of trays can be used in the present process, improvements on theoperation of the present distillation systems, as explained into moredetail below, permit to achieve a similar degree of separation as forcolumns with a higher number of trays. Advantageously, application ofthe present process includes the benefits of less energy usage and lowerconstruction costs.

In an alternative embodiment, one or more of said distillation columnsare divided wall distillation column or divided wall column. Such columnis a distillation vessel having a vertical partition separating one sidefrom the other for a portion or all of the height of the vessel.Although such column comprises a larger number of trays, the use of suchsingle column may be advantageous with regard to construction costs andenergetic requirements.

In a preferred embodiment, one or more of said distillation columns arepacking columns. Packing column refers to column packed with inert solidparticles.

Reboilers are used as heat exchangers to provide heat to the bottom ofsaid distillation columns. They boil the liquid from the bottom of adistillation column to generate vapors which are returned to the columnto drive the distillation separation. The reboiler receives a liquidstream from the column bottom and may partially or completely vaporizethat stream. Steam usually provides the heat required for thevaporization. In an embodiment of the present invention, the hotcondensates from said reboiler are used for driving the absorptionrefrigeration unit for use in the present invention.

This process allows recycling energy from the water condensates fromsaid reboiler by using said energy for driving an absorptionrefrigeration cycle.

Following a polymerization process, polymer effluent is generallyseparated from the liquid by flash vaporization. According to theinvention, the hereby obtained vaporous feed stream, comprising monomer,such as ethylene, co-monomer, such as 1-hexene, and diluent, such asisobutane, is subsequently separated into individual monomer, co-monomerand diluent streams in a separation system comprising one or moredistillation zones.

Separate streams of monomer, co-monomer and diluent are recovered forfurther use, e.g. use in the polymerization reaction. The vaporous feedstream, coming from the flash tanks also comprises traces of bothheavier, e.g. oligomers, and lighter components including N₂, H₂, andlight poisonous components such as O₂, CO and CO₂, and formaldehyde.Such components are herein also denoted as “poisonous components”,because such components are detrimental for the activity of a catalyst.Re-introduction thereof into a polymerization reactor could greatlydisturb catalyst activity and thus reduce polymerization efficiency. Itis therefore of the utmost importance to have a recovery system adaptedto recover essentially pure streams of (co-)monomer, and diluent,without substantial residual amount of such poisonous components forre-use in a polymerization process.

In general, the distillation zone is used for the separation of 3 maincomponents, comonomer, diluent, and monomer. The major component is thediluent, with a part of 90-95 wt % of the feed stream of thedistillation section.

In a first column splits all components heavier than the diluent (inbottom), the top of the column (overhead stream) is sent to a secondcolumn. In an embodiment, the top of the column (i.e. the overheadstream) can be chilled using an absorption refrigeration unit.

The second column removes from the diluent lighter components, in orderto get pure diluent. In an embodiment, the top of the column (i.e. theoverhead stream) can be chilled using an absorption refrigeration unit.

The heavy end and light end can be optionally treated in other columnsdepending on the requests. These two streams represent less than 10% ofthe column feed.

According to an embodiment of the present process the feed stream ispassed to a distillation column and subjected to distillation conditionsadapted to remove a bottom stream comprising substantially olefin-freehydrocarbon diluent, optionally a side stream comprising hydrocarbondiluent, and an overhead vapor stream comprising olefin monomer, diluentand further components such as H₂, N₂, O₂, CO, CO₂ and formaldehyde.

The bottom stream comprises substantially olefin-free hydrocarbondiluent. The term “substantially olefin-free hydrocarbon diluent” or“olefin-free diluent” or the like are used herein as synonyms to denotehydrocarbon diluent which contains less than 5000 ppm, and preferablyless than 1000 ppm, and even more preferred less than 100 ppm of monomerand/or co-monomer. Substantially free of traces of monomer such asethylene and/or co-monomer such as hexene, the bottom stream of olefinfree hydrocarbon diluent, such as isobutane, issued from thedistillation column can be sent to a storage tank and further used, e.g.for flushing conduits and circulation pumps in a polymerization reactor,or for catalyst preparation e.g. in mud pots. This olefin-free diluentcan be recycled to a polymerization zone, whether homo-polymerization orco-polymerization, at any place of the process where pure diluent isrequested, like the catalyst dilution.

The side stream of hydrocarbon diluent issued from the distillationcolumn is generally sent to a storage tank and further used. Preferably,the amount of further components such as H₂, N₂, O₂, CO and CO₂,formaldehyde in the side stream is lower than 10 ppm, and preferablylower than 1 ppm, and even more preferred lower than 0.5 ppm. In anotherpreferred embodiment, the amounts of monomer and/or co-monomer remainingin the side stream are lower than 25% and preferably lower than 10% andeven more preferred lower than 5%. High amounts of monomer in thestorage tank of the side-stream product may lead to evaporation andsubstantial monomer loss. By keeping the amount of monomer in theside-stream product below 25% and preferably below 10%, or even below5%, evaporation of monomer from the storage tank can be reduced andstorage of the side-stream product at atmospheric conditions becomespossible. The hydrocarbon diluent issued from the side stream exitingfrom the distillation zone is generally used as diluent in apolymerization reactor, either homo-polymerization or co-polymerizationdepending upon monomers being subjected to polymerization. It is inparticular very suitable for use as diluent especially in a secondpolymerization reactor when polymerizing under bimodal operation, or ina first as well as a second reactor, when polymerizing under monomodaloperation.

Light components such as formaldehyde, H₂, N₂, O₂, CO and CO₂ exit thedistillation zone with some residual monomer and diluent as an overheadvapor stream. According to an embodiment of the present invention saidoverhead vapor stream is cooled below 0° C., preferably below −10° C.,more preferably between −10° C. and −25° C., yet more preferably between−10° C. and −20° C., more preferably at about −20° C. using anabsorption refrigeration cycle driven by the heat from the reboilerwater condensates. In particular the overhead vapor stream istransferred to a vent condenser which is chilled using an absorptionrefrigeration unit. After chilling in the vent condenser partialcondensation takes places, liquid is sent back to the column (it is thereflux) and the gas is sent to the Ethylene Recovery Unit (ERU). Theselight components are then further treated in the Ethylene Recovery Unit,which further separates the light components from the remaining monomerand hydrocarbon diluent.

The vent condenser is a chilling allowing the separation of thecomponents from the overhead vapor stream, which chilling has beenperformed using the absorption refrigeration unit. The separation isperformed at temperatures preferably below 0° C., preferably between 0°C. and −60° C., preferably between −10° C. and −30° C., more preferablybetween −10° C. and −25° C., yet more preferably between −10° C. and−20° C., more preferably at about −20° C. This separation allows thecondensation and the recovery of the maximum isobutane and minimizes thequantity of isobutane which are conveyed to the ERU (ethylene recoveryunit). This is achieved according to an embodiment of the presentinvention by chilling the stream sent to the ERU using an absorptionrefrigeration cycle.

Under prior art conditions, the stream conveyed to the ERU comprisedisobutane, ethylene, hydrogen, nitrogen and ethane. Ethylene andisobutane were further recovered in ERU. Using the process of theinvention allows reducing the amount of isobutane sent to the ERU, sinceit is almost completely recovered in the vent condenser chilled by theabsorption refrigeration unit.

Preferably, the amount of remaining diluent that is sent to the ERU islower than 30%, preferably lower than 20%, preferably lower than 10%,preferably lower than 5% and more preferably lower than 1%. Preferably,the amount of remaining monomer sent to the ERU is also lower than 50%.Monomer and diluent that are recovered by means of the ERU unit arepreferably re-used in the polymerization process.

In one embodiment, the present process is carried out in a distillationsystem, which comprises one distillation zone or column. Preferably saidcolumn may comprise a divided wall distillation column or divided wallcolumn. In such case, the invention provides for a process foroptimizing the separation of a hydrocarbon-containing feed streamcomprising the steps of:

a) optionally cooling said hydrocarbon-containing feed using anabsorption refrigeration cycle,b) passing said feed to a distillation zone for subjecting said feed todistillation conditions adapted to remove b1) a bottom stream comprisingco-monomer and hydrocarbon diluent, and b2) an overhead vapor streamcomprising hydrocarbon diluent, olefin monomer and further componentssuch as H₂, N₂, O₂, CO, CO₂, and formaldehyde, andc) cooling the temperature of said removed overhead vapor stream usingan absorption refrigeration cycle prior to separating said olefinmonomer and diluent in overhead said vapor stream.

In another embodiment, the present process is carried out in adistillation system, which comprises at least two distillation zones orcolumns. In such case, the invention provides for a process foroptimizing the separation of a hydrocarbon-containing feed streamcomprising the steps of:

a) optionally cooling said hydrocarbon-containing feed stream using anabsorption refrigeration cycle,b) passing said feed stream to a first distillation zone for subjectingsaid feed to distillation conditions adapted to remove b1) a bottomstream comprising co-monomer, and b2) an overhead stream comprisinghydrocarbon diluent, olefin monomer and further components such as H₂,N₂, O₂, CO, CO₂, and formaldehyde, andc) introducing the overhead stream of step b) in a second distillationzone for subjecting said stream to distillation conditions adapted toremove c1) a bottom stream comprising substantially olefin-freehydrocarbon diluent, c2) a side stream comprising hydrocarbon diluent,and c3) an overhead vapor stream comprising olefin monomer, diluent andfurther components such as formaldehyde, H₂, N₂, O₂, CO and CO₂, andd) cooling the temperature of said removed overhead vapor stream usingan absorption refrigeration cycle prior to separating said olefinmonomer and diluent in said overhead vapor stream.

In yet another embodiment, the present process is carried out in adistillation system, which comprises three distillation zones orcolumns. In such case, the invention provides for a process foroptimizing the separation of a hydrocarbon-containing feed streamcomprising the steps of:

a) optionally cooling said hydrocarbon-containing feed stream using anabsorption refrigeration cycle,b) passing said feed stream to a first distillation zone for subjectingsaid feed to distillation conditions adapted to remove b1) a bottomstream comprising co-monomer and hydrocarbon diluent and b2) an overheadstream comprising hydrocarbon diluent, olefin monomer and furthercomponents such as H₂, N₂, O₂, CO, CO₂ and formaldehyde,c) introducing the bottom stream of step b) in a second distillationzone for subjecting said stream to distillation conditions adapted toremove c1) a bottom stream comprising co-monomer and c2) an overheadstream comprising hydrocarbon diluent,d) introducing the overhead stream of step b) in a third distillationzone for subjecting said stream to distillation conditions adapted toremove d1) a bottom stream comprising substantially olefin-freehydrocarbon diluent, d2) a side stream comprising hydrocarbon diluent,and d3) an overhead vapor stream comprising olefin monomer, diluent, andfurther components such as formaldehyde, H₂, N₂, O₂, CO and CO₂, andd) cooling the temperature of said removed overhead vapor stream of step(c) using an absorption refrigeration cycle prior to separating saidolefin monomer and diluent in said overhead vapor stream.

The hydrocarbon diluent obtained in stream c2) may be returned to thefirst distillation zone.

The above-described processes according to the present invention can befurther adapted.

Said adaptation may consists in maximizing the overhead stream (e.g. c3)or d3)) obtained from the distillation column by adapting the ratio ofthe bottom stream flow removed from the distillation column to the feedstream flow introduced in the distillation column. In an embodiment,said ratio is lower than or equal to 1.0, and preferably comprisedbetween 0.3 and 1.0, and more preferably between 0.4 and 0.95, and mayfor instance comprise 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9. The ratio setpoint of bottom stream flow rate to feed stream flow rate which issuitable according to the present process can be adjusted manually orautomatically, to ensure enough olefin free diluent product and suitableventing to the ERU to remove light components. The ratio can be adaptedby adapting the reflux to the distillation column.

Said adaptation may also consist in adding the side stream obtained inthe distillation column with the bottom stream obtained in thedistillation column. Generally, the bottom stream of olefin freehydrocarbon diluent issued from the distillation column, and the sidestream of hydrocarbon diluent are both sent to separate storage tanks.Diluting the side stream with the bottom stream permits to control thelevel of product in the storage tanks for the side-stream product andfor the bottom-stream product. Diluting the side stream with the bottomstream is further required in view of controlling the ratio of thebottom stream flow to the feed stream flow.

Other improvements to the operation of the distillation column includeimprovements for stabilizing the distillation conditions in thedistillation column.

For that, a further step can be provided of re-boiling a portion of thebottom stream obtained in the distillation column under controlled steamflow rate and returning said re-boiled portion to the distillationcolumn.

The rate of the steam flow can be controlled as a function of thetemperature in the distillation column. Preferably, the steam flow rateis controlled as a function of the temperature on a tray located in thelower half of the column, i.e. equal to or lower than ½ of the height ofthe column, and even more preferred located in the lower quarter of thecolumn, i.e. equal to or lower than ¼ of the height of the column.Preferably, this temperature is used as an input parameter in atemperature controller system driving and controlling the reboiler steamflow rate. In addition, in order to make the control system asinsensitive as possible to pressure variations in the distillation zone,the controller parameters have been chosen so as to obtain a relativelyslow reaction of the controller system. In addition, the sump level ofthe distillation column can be adapted to the bottom stream flow bymeans of a level controller.

In another aspect, the present invention also provides a polyolefinproduction process, comprising the steps of: introducing into a reactorone or more olefin reactants, polymerization catalysts and diluents, andwhile circulating said reactants, catalysts and diluents, polymerizingone or more olefin reactants to produce a polymer slurry comprisingessentially liquid diluent and solid olefin polymer particles,recovering olefin polymer particles from the slurry by separating atleast a majority of the diluent from the slurry in ahydrocarbon-containing feed stream, separating saidhydrocarbon-containing feed stream, removing at least one stream fromsaid separation step, and cooling the temperature of said removed streamusing an absorption refrigeration cycle.

In one embodiment, said removed stream is an overhead vapor streamremoved from a distillation step, said stream comprising olefin monomerand diluent. In another embodiment, said removed stream is a side streamcomprising hydrocarbon diluent removed from a distillation step. Inanother embodiment, both said overhead vapor stream and said side streamare refrigerated using at least two absorption refrigeration units.

In another embodiment, the temperature of the hydrocarbon-containingfeed stream is cooled using at least one absorption refrigeration cycle.

In an embodiment, the present invention also provides a polyolefinproduction process, comprising the steps of: introducing into a reactorone or more olefin reactants, polymerization catalysts and diluents, andwhile circulating said reactants, catalysts and diluents, polymerizingone or more olefin reactants to produce a polymer slurry comprisingessentially liquid diluent and solid olefin polymer particles,recovering olefin polymer particles from the slurry by separating atleast a majority of the diluent from the slurry in ahydrocarbon-containing feed stream, separating saidhydrocarbon-containing feed stream, removing an overhead vapor streamfrom said separation step, and cooling the temperature of said removedoverhead vapor stream using an absorption refrigeration cycle prior toseparating said olefin monomer and diluent in said removed vapor stream.

The invention also provides a system for cooling an overhead vaporstream removed from a distillation column using an absorptionrefrigeration unit. Said absorption refrigeration unit is used to removethermal energy from said overhead vapor stream and phase conversion of aworking fluid is used in the heat removal process.

The present invention also provides one or more diluent/monomer recoverysystem configured to separate a majority of the diluent from the slurrydischarged from the polymerization reactor; said system comprising

one or more fractionation system configured to process a portion of thediluent discharged from the diluent/monomer recovery system and toprovide recovered diluent substantially free of olefin monomer; whereinsaid one or more fractionation system is coupled with one or moreabsorption refrigeration unit.

In a preferred embodiment, said polyolefin producing unit, comprises:

-   -   means for feeding monomer, a co-monomer, diluent and optionally        hydrogen to at least one polymerization reactor;    -   means for feeding a polymerization catalyst in said at least one        polymerization reactor,    -   a reactor system comprising at least one polymerization reactor        defining a flow path for a polymer slurry, said slurry        consisting essentially of at least one monomer, a co-monomer, a        polymerization catalyst, liquid diluent and solid olefin        co-polymer particles,    -   one or more lines for discharging said polymer slurry out of        said polymerization reactor,    -   one or more diluent/monomer recovery system configured to        separate a majority of the diluent from the slurry discharged        from the polymerization reactor;    -   one or more fractionation system configured to process a portion        of the diluent discharged from the diluent/monomer recovery        system and to provide recovered diluent substantially free of        olefin monomer; and    -   an extrusion/loadout system having an extruder/pelletizer        configured to extrude and pelletize polyolefin particles        recovered from the slurry in the diluent/monomer recovery        system,        wherein said one or more fractionation system is coupled with        one or more absorption refrigeration unit.

A recycling unit according to an embodiment of the invention isschematically illustrated in FIG. 1A. Said recycling unit comprises adistillation column 2, whose overhead vapor line is in thermalcommunication with an absorption refrigeration unit 4. Said overheadvapor line is further in communication with a separator 6. Thehydrocarbon-containing feed stream 1 that is to be separated willgenerally be an overhead stream coming from a flash tank and purgecolumns of a polymerization reactor, wherein a stream containing solvent(solvent=diluent), polymer and unreacted monomers is flashed orotherwise treated to remove solvent or diluent and monomers there from.A presently preferred component stream separated according to theinvention comprises monomer, such as ethylene, co-monomer, such as1-hexene, and diluent, such as isobutane. It should be recognizedhowever, that the distillation system of the invention is equallyapplicable to other monomer, co-monomer and diluent systems so long asfeed vapors comprise hydrocarbons, which permit separation bydistillation. Traces of both heavier, e.g. oligomers, and lightercomponents such as formaldehyde, N₂, H₂, and components such as O₂, COand CO₂ are generally also present in such effluent streams. Thehydrocarbon-containing feed stream 1 is fed to said distillation column2. A bottom stream 9 comprising comonomer and hydrocarbon diluent isremoved from said distillation column 2 and further recovered 13. In anembodiment, said hydrocarbon-containing feed stream 1 is an overheadvapor stream removed from a first distillation zone 10, and the bottomstream 9 comprises substantially olefin-free hydrocarbon diluent. Anoverhead vapor stream 3 comprising olefin monomers and diluent isremoved from said distillation column 2 and cooled in a condenser (notshown) by an absorption refrigeration unit 4. In an embodiment, theoverhead vapor stream 3 enters the condenser chilled by the absorptionrefrigeration unit 4 at for example about 28° C. and is cooled totemperature below 0° C. preferably, between −10° C. and −30° C. Thecooled overhead vapor stream 5 is then sent to the vent condenser 6where the ethylene and liquid isobutane 8 are recovered 12, while theoverhead vapor stream 7 from the condenser 6 are sent to the ERU 11. Thereboiler of the column 2 (not shown) can be thermally connected to theabsorption refrigeration unit 4, providing the heat to drive saidrefrigeration unit 4. The absorption refrigeration unit 4 can also bepowered by an alternative hot source (not shown).

A recycling unit according to another embodiment of the invention isschematically illustrated in FIG. 1B. Said recycling unit comprises adistillation column 102, whose overhead vapor line is in thermalcommunication with chillers 107 and 105. Said overhead vapor line isfurther in communication with a separator 106. An absorptionrefrigeration unit 104 is connected between the chiller 105 and thecolumn boiler 103. The hydrocarbon-containing feed stream 100 that is tobe separated will generally be an overhead stream coming from a flashtank and purge columns of a polymerization reactor, wherein a streamcontaining solvent (solvent=diluent), polymer and unreacted monomers isflashed or otherwise treated to remove solvent or diluent and monomersthere from. The hydrocarbon-containing feed stream 100 is fed to saiddistillation column 102. In an embodiment, said hydrocarbon-containingfeed stream 100 is an overhead vapor stream removed from a firstdistillation zone. A bottom stream 121 comprising comonomer andsubstantially olefin-free hydrocarbon diluent is removed from saiddistillation column 102 and further recovered. An overhead vapor stream112 comprising olefin monomers and diluent is removed from saiddistillation column 102 and cooled by an absorption refrigeration unit104 in the condenser 105. In an embodiment, the overhead vapor stream112 enters first chiller 107, and the chilled overhead vapor stream 113enters the condenser 105 at for example about 28° C. and is cooled totemperature below 0° C. preferably, between −10° C. and −30° C. Thecooled overhead vapor stream 114 is then sent to the separator 106 wherethe ethylene and liquid isobutane 116 are recovered and recycled to thereactor (not shown) and partly recycled 122 to the column 102, while theoverhead vapor stream 115 from the separator 106 is sent to the ERU(ethylene recovery unit). The reboiler 103 of the column 102 isthermally connected 117 to the absorption refrigeration unit 104,providing the heat to drive said refrigeration unit 104. A stream ofcoolant fluid circulates between absorption unit 104 and condenser 105.In the case of an ammonia system, liquid ammonia 119 is fed to thecondenser 105, and gaseous ammonia 120 is redirected to the absorptionunit 104.

As illustrated in FIG. 2A, a recycling unit according to an embodimentof the invention can be composed of two distillation columns 22, 23, andof an ethylene recovery unit represented by the arrow 35. Thehydrocarbon-containing feed stream 20 to be separated will generally bean overhead stream coming from a flash tank and purge columns of apolymerization reactor (not shown), wherein a stream containing solvent,polymer and unreacted monomers is flashed or otherwise treated to removesolvent or diluent and monomers therefrom. A first distillation column22 realizes a rough cut between a mixture of isobutane, hexene and theheavies, which exit as liquid bottom product 25. The heavy bottomproduct 25 can be further treated (not shown). The remaining monomer,isobutane, with all light components, 29 which exits from the top of thefirst distillation column 22, is sent to a second distillation column 23as a vapor stream for further separation. The second distillation column23 will generate three product streams. More in particular, this column23 is used to separate olefin-free isobutane diluent from isobutanediluent containing residual amounts of (co-)monomer and from a lightvapor stream comprising monomer, additional residual isobutane andfurther components such as formaldehyde, H₂, N₂, O₂, CO and CO₂.Substantially pure isobutane, so-called “substantially olefin-free”isobutane is obtained as liquid bottom product 30. Light components suchas formaldehyde, H₂, N₂, O₂, CO and CO₂ exit the distillation column 23with ethylene and some residual isobutane as a vapor stream 32, whichaccording to an embodiment of the invention are further cooled using anabsorption refrigeration unit 34 before being further purified andseparated in an ethylene recovery unit represented by the arrow 35. Inan embodiment, the overhead vapor stream 32 enters a condenser chilledby the absorption refrigeration unit 34 at for example about 28° C. andis cooled to temperature below 0° C. preferably, between −10° C. and−30° C. The cooled overhead vapor stream is then sent to an ethylenerecovery unit 35. Said ethylene recovery unit preferably comprising avent condenser (not shown) where the ethylene and liquid isobutane areseparated, and the overhead vapor stream from the vent condenser is sentto the ERU (ethylene recovery unit). In an embodiment, the ethylenerecovery unit 35 can be avoided. The chilling of stream 32 withabsorption refrigeration unit 34 decreases dramatically the stream to befurther separated, leading to a competitive process in terms of ethylenerecovery without ethylene recovery unit.

In an embodiment, the reboiler (not shown) of the first column 22 can bethermally connected to the absorption refrigeration unit 34, providingthe heat to drive said refrigeration unit 34. The absorptionrefrigeration unit can also be powered by an alternative hot source.

Remaining isobutane exits the column 23 as a liquid side stream 31. Thedistillation process in the second distillation column thus permits toseparate substantially olefin-free isobutane diluent in a bottom stream30 as well as isobutane diluent containing residual amounts of ethylenein a side stream 31. Both the substantially olefin-free isobutanediluent 30 and the isobutane diluent 31 can be recycled and re-used in apolymerization process. In addition, isobutane diluent and ethylenemonomer which are separated from the vapor stream 32 are also recycledand re-used in a polymerization process. In another embodiment, thehydrocarbon-containing feed stream 20 that is to be separated can alsobe cooled prior to distillation using at least one absorptionrefrigeration unit 37, said cooled feed stream 21 can then be fed to thedistillation column 22.

As illustrated in FIG. 2B, a recycling unit according to anotherembodiment of the invention can be composed of two distillation columns150, 151 and of an ethylene recovery unit represented by the arrow 168.The hydrocarbon-containing feed stream 158 to be separated willgenerally be an overhead stream coming from a flash tank and purgecolumns of a polymerization reactor (not shown). A first distillationcolumn 150 realizes a rough cut between a mixture of isobutane, hexeneand the heavies, which exit as liquid bottom product 161. The heavybottom product 161 can be further treated (not shown). The remainingmonomer, isobutane with all light components 160, which exits from thetop of the first distillation column 150, is sent to a seconddistillation column 151 as a vapor stream 160 for further separation.The second distillation column 151, —whose overhead vapor line is inthermal communication with chillers 152 and 153—will generate threeproduct streams. Substantially pure isobutane, so-called “substantiallyolefin-free” isobutane is obtained as liquid bottom product 164. Lightcomponents such as formaldehyde, H₂, N₂, O₂, CO and CO₂ exit thedistillation column 151 with ethylene and some residual isobutane as avapor stream 162, which according to an embodiment of the invention isfurther cooled using an absorption refrigeration unit 154. Saidabsorption refrigeration unit 154 is connected between the chiller 153and the column boiler 156 of the first column 150. In an embodiment, theoverhead vapor stream 162 enters first chiller 152, and the chilledoverhead vapor stream 165 enters the condenser 153 at for example about28° C. and is cooled to temperature below 0° C. preferably, between −10°C. and −30° C. The cooled overhead vapor stream 169 is then sent to aseparator 155 where the ethylene and liquid isobutane 167 are separated,and the overhead vapor stream from the separator 166 is sent to the ERU168.

In an embodiment, the ethylene recovery unit 168 can be avoided. Thechilling of stream 165 with absorption refrigeration unit 154 decreasedramatically the stream 166, leading to a competitive process in termsof ethylene recovery without ethylene recovery unit.

In this embodiment, the reboiler 156 of the first column 150 isthermally connected to the absorption refrigeration unit 154, providingthe heat to drive said refrigeration unit 154. The absorptionrefrigeration unit can also be powered by an alternative hot source.Remaining isobutane exits the column 151 as a liquid side stream 163.The distillation process in the second distillation column thus permitsto separate substantially olefin-free isobutane diluent in a bottomstream 164 as well as isobutane diluent containing residual amounts ofethylene in a side stream 163. Both the substantially olefin-freeisobutane diluent 164 and the isobutane diluent 163 can be recycled andre-used in a polymerization process. In an embodiment, thehydrocarbon-containing feed stream 158 that is to be separated can alsobe cooled prior to distillation using at least one absorptionrefrigeration unit 157, said cooled feed stream 159 can then be fed tothe distillation column 150.

As illustrated in FIG. 3, a recycling unit according to an embodiment ofthe invention can be composed of three distillation columns 220, 230,240, in addition to an ethylene recovery unit represented by the arrow350. The hydrocarbon-containing feed stream 200 that is to be separatedwill generally be an overhead stream coming from a flash tank and purgecolumns of a polymerization reactor, wherein a stream containingsolvent, polymer and unreacted monomers is flashed or otherwise treatedto remove solvent or diluent and monomers therefrom. A firstdistillation column 220 realizes a rough cut between a mixture ofisobutane, hexene and the heavies, which exit as liquid bottom product250. The heavy bottom product is further treated in a seconddistillation column 240 and separated into three product streams.Isobutane vapor exiting as top product 260 makes the feed stream of thefirst column 220, or is recycled to a polymerization zone. A purifiedliquid hexene stream 270 is recovered from a tray just above the columnsump and sent to storage for recycling to the polymerization reactor(s).The heavy components 280 are recovered from the column 240 sump with thedraining procedure being triggered on high column bottoms temperature.The remaining monomer, isobutane, with all light components, which exitsfrom the top 290 of the first distillation column 220, is sent to athird distillation column 230 as a vapor stream for further separation.The third distillation column 230 is used to generate three productstreams. More in particular, this column 230 is used to separateolefin-free isobutane diluent from isobutane diluent containing residualamounts of (co-)monomer and from a light vapor stream comprisingmonomer, additional residual isobutane and further components such asformaldehyde, H₂, N₂, O₂, CO and CO₂. Substantially pure isobutane,so-called “substantially olefin-free” isobutane is obtained as liquidbottom product 300. Light components such as formaldehyde, H₂, N₂, O₂,CO and CO₂ exit the distillation column 230 with ethylene and someresidual isobutane as a vapor stream 320, which according to anembodiment of the invention are further cooled using an absorptionrefrigeration unit 340 before being further purified and separated in anethylene recovery unit represented by the arrow 350. Remaining isobutaneexits the column 230 as a liquid side stream 310. The distillationprocess in the third distillation column thus permits to separatesubstantially olefin-free isobutane diluent in a bottom stream as wellas isobutane diluent containing residual amounts of ethylene in a sidestream. Both the substantially olefin-free isobutane diluent and theisobutane diluent are recycled and re-used in a polymerization process.In addition, isobutane diluent and ethylene monomer which are separatedfrom the vapor stream 302 are also recycled and re-used in apolymerization process. In another embodiment, thehydrocarbon-containing feed stream 200 that is to be separated can alsobe cooled using at least one absorption refrigeration unit 370, saidcooled feed stream 210 can then be fed to the distillation column 220.The reboiler (not shown) of the column 240 is preferably thermallyconnected to the absorption refrigeration unit 340, providing the heat(heat of condensates) to drive said refrigeration unit 340. Theabsorption refrigeration unit can also be powered by an alternative hotsource.

The present invention therefore also provides a polyolefin productionprocess, comprising the steps of

-   -   introducing into a reactor one or more olefin reactants,        polymerization catalysts and diluents, and while circulating        said reactants, catalysts and diluents,    -   polymerizing one or more olefin reactants to produce a polymer        slurry comprising essentially liquid diluent and solid olefin        polymer particles,    -   recovering olefin polymer particles from the slurry by        separating at least a majority of the diluent from the slurry in        a hydrocarbon-containing feed stream,    -   distilling said hydrocarbon-containing feed stream,    -   removing an overhead vapor stream comprising olefin monomer and        diluent from said distillation step,    -   distilling said removed overhead vapor stream,    -   removing a second overhead vapor stream and    -   cooling the temperature of said removed second overhead vapor        stream using an absorption refrigeration cycle prior to        separating said olefin monomer and diluent in said removed vapor        stream.

In an embodiment, said process comprises the steps of cooling thetemperature of said hydrocarbon-containing feed stream, using anabsorption refrigeration cycle prior to said first distillation.

While the invention has been described in terms of the presentlypreferred embodiment, reasonable variations and modifications arepossible by those skilled in the art and such variations are within thescope of the described invention and the appended claims.

This invention can be further illustrated by the following examples ofpreferred embodiments thereof, although it will be understood that thisexample is included merely for purposes of illustration and are notintended to limit the scope of the invention unless otherwisespecifically indicated.

EXAMPLES Example 1

An absorption refrigeration unit is provided between a distillationcolumn and a vent condenser in a recycling unit of an olefinpolymerization process, preferably a polyethylene production process.The absorption refrigeration units are powered by low pressure steam orhot water providing from the reboiler of a distillation column. Theexample developed here relies on ammonia-water system. Absorptionrefrigeration units with ammonia are able to produce cold source between0° C. and −60° C. (negative cold), preferably between 0° C. and −30° C.,more preferably between −10° C. and −20° C., such that the overheadvapor stream directed to the vent condenser and consequently to the ERU(Ethylene recycling unit) is at temperatures below 0° C. This system hasthe further advantage that is uses hot water that is available in thevicinity of the condenser, for instance the condensates from thereboiler of the distillation column.

A recycling unit according to an embodiment of the invention isillustrated in FIG. 1A, wherein a separation column 2 and a “ventcondenser” 6 are indicated and the absorption refrigeration unit 4 isprovided between said separation column 2 and the vent condenser 6. Thisrecycling unit setup can be used to recover a large part or most of theisobutane. The overhead vapor stream 3 enters the absorptionrefrigeration unit 4 at about 28° C. and is cooled to temperature below0° C. preferably, between −10° C. and −20° C. The cooled overhead vaporstream 5 is then sent to the vent condenser 6 where the ethylene andliquid isobutane 8 are recovered 12, while the overhead vapor stream 7from the condenser 6 are sent to the ERU 11. Table 1 shows thecomposition of the streams in said recycling unit.

TABLE 1 Number on Stream FIG. 1A Composition hydrocarbon-containing feed1 C₂ ⁼, iC₄, ε light, stream ε C₆ ⁺, ε H₂ . . . overhead vapor stream 3= 5 light, C₂ ⁼, iC₄, ε H₂ bottom stream 9 iC₄ overhead stream from thevent 7 C₂ ⁼, iC₄, H₂, N₂, C₂ condenser bottom stream from the vent 8iC₄, C₂ ⁼, C₂ condenser ε means a small quantity

As illustrated in Table 2, the advantage of cooling the overhead vaporstream from the separation column permit to condense and recover themaximum of isobutane and therefore minimizing the quantity of isobutanesent to the ERU. Table 6 shows the results when cooling the overheadstream below 0° C. according to an embodiment of the invention.

TABLE 2 C₂ (Stream 7) iC₄ (Stream 7) T (° C.) (kg/h) (kg/h) % Δ (C₂) % Δ(iC₄) −60 182 2.9 69 98.8 −50 316 7.6 47 96.7 −40 478 21 19 91.2 −30 53945 9 81 −20 564 83 5 64.6 −10 579 140 2.4 40 1 593 235 0 0

The use of said refrigeration unit for the chilling below 0° C. theoverhead vapor stream coming from the distillation column decreases thequantity of isobutane that is sent to the ERU as shown in Table 2.

Example 2

Another embodiment of the invention is described in this example. Thehydrocarbon-containing feed stream providing from the flash tankdownstream a polymerization reaction is cooled using an absorptionrefrigeration unit positioned before a first separation column. In therecycling unit the efficiency of two-stage compressors of the isobutanerecycling stream (hydrocarbon containing feed stream) is improved by theinstallation of an absorption refrigerating unit for cooling saidisobutane stream.

This embodiment according to the invention comprises providing oneabsorption refrigeration unit in front of the first stage of thecompressors, and one absorption refrigeration unit between the twostages. These enable the gas at the intake of the first stage to becooled, and then the gas leaving the first stage is further cooled bythe second refrigeration unit. This configuration allows the increase ofthe capacity of the compressors by at least 16.5%. In an embodiment apurge system is further provided between the second refrigeration unitand the second stage compressor.

Example 3

Another embodiment of the invention is described in this example whereina side stream removed from a distillation column is cooled using anabsorption refrigeration unit. Water and lithium bromide are used asworking fluids in said absorption refrigeration cycle.

The cooling of this side streams prevents any pressure increase of themixture of olefins and diluent. Since this stream is recycled to thereactor, a chilling of this stream also contributes to the reactorrefrigeration. A temperature decrease of this stream by 20° C. increasesthe cooling capacity of the polymerization reactor by 1%.

1-11. (canceled)
 12. A process for the separation of ahydrocarbon-containing feed stream comprising: cooling thehydrocarbon-containing feed stream using a first absorptionrefrigeration cycle to form a cooled feed stream; introducing the cooledfeed stream into a first distillation zone for subjecting the cooledfeed stream to distillation conditions adapted to remove a first bottomstream comprising co-monomer and a first overhead stream comprisinghydrocarbon diluents, olefin monomer and further components selectedfrom H₂, N₂, O₂, CO, CO₂, formaldehyde and combinations thereof; passingthe overhead stream from the first distillation zone to a seconddistillation zone for subjecting the overhead stream to distillationconditions adapted to remove a second bottom stream comprisingsubstantially olefin-free hydrocarbon diluents, a side stream comprisinghydrocarbon diluent and a second overhead vapor stream comprising olefinmonomer, diluents and further components selected from H₂, N₂, O₂, CO,CO₂, formaldehyde and combinations thereof; cooling the second overheadvapor stream using a second absorption refrigeration cycle to form acooled overhead vapor stream; and separating olefin monomer fromdiluents in the cooled overhead vapor stream.
 13. The process of claim12, wherein the cooled overhead vapor stream has a temperature ofbetween 0° C. and −40° C.
 14. The process of claim 12, wherein thesecond absorption refrigeration cycle utilizes water and ammonia asworking fluids.
 15. The process of claim 12, wherein the secondabsorption refrigeration cycle is driven from heat recycled from areboiler of the first distillation zoned.
 16. The process of claim 12further comprising cooling the side stream using a third absorptionrefrigeration cycle.
 17. The process of claim 16, wherein the thirdabsorption refrigeration cycle utilizes water and lithium bromide asworking fluids.
 18. The process of claim 16, wherein the thirdabsorption refrigeration cycle utilizes water and ammonia as workingfluids.
 19. The process of claim 12, wherein the first absorptionrefrigeration cycle utilizes water and lithium bromide as workingfluids.
 20. The process of claim 12 further comprising passing the firstbottom stream from the first distillation zone to a third distillationzone for subjecting the first bottom stream to distillation conditionsadapted to remove a third bottom stream comprising co-monomer and athird overhead stream comprising hydrocarbon diluents.
 21. The processof claim 12, wherein the hydrocarbon-containing feed stream is a productof a polyolefin production process.